I have recently purchased one & I am impressed with the build quality & weight:size ratio. It doesn't make the set up muzzle heavy at all. I have yet to fire it & test for sound / recoil reduction but hope to do so in the next couple of weeks.
I used an SL5 on two .308's for several seasons. Brilliant mod. It may not be the quietest but without measuring the decibels it would be difficult IMO to tell the difference between it and any other I've used. The build quality coupled with the stainless steel materiel is excellent.
I am picking up another new SL5i tomorrow for a 6.5x47 Lapua and a .308 (same thread so it will be a 30cal moderator to use on both). The new one (SL5i) is 60 grammes lighter than my previous SL5 and only adds 103mm to the length (I think???). That's no longer then just about any over the barrel mod.
I was recently talking to a rifle builder who said he's not fan of the reflex style of mods as they retain the gases for longer. I don't use one to get the absolute maximum sound reduction, as they all still have a 'crack'! It's primarily used to reduce recoil and quieten enough to avoid damaging my hearing.
I've looked at a lot of mods recently and spoken to a few mates I know in the industry, I will keep using the SL5(i) in the meantime.
I've had SL5 mods for years, currently on two .308s, one 20" one 24", but now need a new mod for upcoming rebarrel to 6.5x47 which will have heavier barrel and hence different thread. Possibly not the quietest, but nigh on maintenance free and a nice wee compact lump to reduce flip. Hoping the "i" version is an improvement on what was a great thing anyway.